1. Field of the Invention
The present invention relates to a charge coupled device (CCD) and a method for manufacturing the same, and more particularly, to a CCD having a small coupling capacitance between transfer electrodes which CCD is used in an area solid image pick-up element, linear solid state image pick-up element, memory and delay element, and a method for manufacturing the CCD.
2. Description of Related Art
In a solid state image pick-up device, high speed charge transfer is recently required as the pixel density is increased. In order to achieve such high speed charge transfer, it is extremely importance to reduce a coupling capacitance between transfer electrodes.
FIG. 1A is a plan view of an area solid image pick-up element using a conventional CCD and FIG. 1B is a cross sectional view of the image pick-up element cut along the line B--B'. Transfer electrodes are usually formed using two or three layers of polysilicon film. In this example, two layers of polysilicon film are used. In FIG. 1A, a photo-electric converting section 110 generates electric charges in accordance with incident light and stores them. A buried channel layer 102 is provided under a first layer transfer electrode 105 of polysilicon film and a second layer transfer electrode 106 of polysilicon film. The charges stored in the photo-electric converting section 110 are read out into the channel layer 102 under the second layer transfer electrode 106 by applying a high voltage of 10 V or more to the second layer transfer electrode 106 for every predetermined time period. Then, the read out charges are transferred downward in FIG. 1A by applying transfer pulses to the transfer electrodes 105 and 106.
As shown in FIG. 1B, the buried channel layer 102 is formed in a predetermined region on a P-type silicon substrate 101. N-type impurity is doped into the buried channel layer 102. The buried channel layer 102 is widely used in CCD registers. The first layer transfer electrodes 105 are arranged with a predetermined pitch via a gate oxide film 104 on the surface of the semiconductor substrate 101 in which the buried channel layer 102 has been formed. The second layer transfer electrodes 106 are each arranged between the first layer transfer electrodes 105. In order to prevent the decrease of transfer efficiency, it is necessary to provide the transfer electrodes very closely to each other. However, for reason of the manufacturing precision, the second layer transfer electrode 106 is provided to override on the first layer transfer electrode 105 at the end portion.
In the above-mentioned conventional example, since two layers of polysilicon electrodes are formed in such a manner that the two layers overlap at the end portion, a coupling capacitance is great between the first layer transfer electrode 105 and the second layer transfer electrode 106. If the coupling capacitance is great, power consumption increases. At the same time, the load of a driver for driving the device is heavy, so that it is difficult to drive the device at a high speed. Further, there is a large step at the overlapping portion of the first layer transfer electrode 105 and the second layer transfer electrode 106. For this reason, processing to the step portion is difficult in the photolithography process so that the processing precision is further decreased. In addition, in a case where the device is used for the solid state image pick-up device, light leaks into the channel layer of the CCD from the gap produced due to the step portion under a light shielding film, the smear characteristic is degraded.
For solving the above-mentioned problems, a method is proposed in which one layer polysilicon electrode material is etched using a resist mask to form a pattern of transfer electrodes which are repeatedly arranged with a narrow space, as shown in "A Low Driving Voltage CCD with Single Layer Electrode Structure for Area Image Sensor" (IEDM Tech. Dig., pp. 705-708, 1994). The gap between the transfer electrodes is required to be 0.2 .mu.m or below for transferring charges between the CCD registers with an efficiency close to 100%. However, it is difficult to dispose the gap with such a precision by lithography technique currently used for mass-production.